The present invention relates to a current supply device for movable electrodes, particularly running wire electrodes of spark erosion machines.
For spark erosive machining and allied processes, ever higher pulse energies are used, which leads to unacceptably short current supply element life periods. As a result of the high current loading of the current supply elements, a material abrasion takes place, which causes executive wear of current supply elements.
In the case of wire erosion machines the current is fed to the wire electrode normally by means of two current supply elements, one being positioned above and the other below the workpiece. DE-OS 26 53 857 (corresponding to U.S. Pat. No. 4,205,212 has already proposed using two or more current supply elements for each workpiece side, as well as constructing the pulse generator (current source) in a two-channel manner, to improve the current distribution. JP-OS 62-34724 discloses a current supply device with at least two contacts and two generator channels, a higher voltage being applied to the contact more remote from the workpiece.
JP-OS 61-236433 also discloses several successively arranged current supply contacts, the contacts closer to the workpiece being in each case wired to a diode, in order to achieve a better current distribution.
However, all these measures have failed to lead to desired results. Thus, use is now made of multiply usable, single current supply elements, which have to be moved or turned after every couple of hours in operation. This will not be acceptable in future in the case of automatic longterm use of installations. A multi-channel construction of the pulse generators is very costly, particularly if the polarity of the pulses has to be reversed for special machining operations. Modern pulse generators, like those described e.g. in DE-OS 36 39 256, produce pulse currents up to 1000A. Thus, an unnecessary pulse loss capacity of 1 KW is produced per volt of unnecessarily present voltage drop. The two aforementioned Japanese specifications were unable to solve this problem, because the voltage drops produced along the electrode due to the pulse current are of an ohmic and inductive nature. Thus, they are dependent on the pulse current, its derivation after time and the ohmic and inductive components of the electrode. Thus, the voltage drops are not constant, so that the very idea of compensating them with constant voltages is wrong from the outset.